The present invention is related to devices for metering the rate of single-phase gas flow in gas pipes and, more particularly, to such metering devices for use in landfills.
Landfills are a valuable source of methane gas. Waste products from households and businesses, which over time accumulate in landfills, initially decompose under aerobic conditions, under which the composition requires free oxygen. After substantially all of the available free oxygen has been consumed, decomposition becomes anaerobic, at which point methane and carbon dioxide are generated.
In its simplest form, methane may be recovered from the landfill by means of a well casing, typically a cylindrical pipe, which is sunk into the landfill substantially to its bottom. The casing is perforated toward its bottom and suction is applied at its top so as to impose a pressure gradient in the landfill, with pressure diminishing from an area around the casing toward the casing itself. In this way, methane is withdrawn by suction from a field which surrounds the casing through the casing's perforations and up through the casing by advection.
There is a limit to the size of a field that can be served by a single casing. The limiting factor is the maximum allowable air inflow into the landfill which is induced by the advection created by the suction process. If the rate of flow is increased beyond a certain limit, air flow, and hence intrusion of oxygen into the landfill, becomes excessive, in that decomposition becomes again at least partly aerobic, an undesirable condition.
To overcome the above limitation, large landfills are typically served by a network of well casings, each serving a limited region, with individual casings in the network typically being connected through lateral pipes to one or more common blowers/compressors which create the suction necessary to maintain gas flow from the landfill up through the individual well casings. In order to ensure optimum pressure gradients throughout the landfill, it is necessary to monitor the rate of gas flow through the individual well casings. Three techniques for measuring such gas flow may be identified: 1) the uniform long pipe (ULP), 2) velocity pressure metering, and 3) the orifice meter.
The ULP technique involves a long pipe in which gas pressure is measured at widely separated positions so as to detect the pressure drop between them. Since the pressure drop between the two positions will be proportional to the distance between them, and inversely proportional to the cross-sectional area of the pipe, the necessary length of pipe will be dictated by the flow rate, by the sensitivity of the pressure sensors, and by the cross-sectional area of the pipe. In order to obtain a measurable pressure drop, the length of the ULP is typically many times its diameter. This creates a placement problem.
Velocity pressure metering senses gas velocity at a point within a pipe by sensing total gas pressure at that point and static pressure at or near that point and effectively subtracting static from total pressure to yield velocity pressure. From velocity pressure, gas flow rate may be readily derived. The total and static pressure pickups may be either separate or integrated into a common pitot tube, a well-known pressure sensor. A pitot tube, for measuring gas pressure within a pipe, typically comprises a pair of concentric tubes extending axially along the interior of the pipe. The inner tube has an opening facing upstream, and the outer tube has an opening oriented perpendicular to the direction of flow. Total pressure is sensed through the inner opening; static pressure through the side opening. Velocity pressure at the tip of the concentric pipes is derived from the difference between the two pressures. In order to eliminate disturbances in gas flow at the point of measurement, and create a smooth, concentrically symmetric flow regime, it is necessary to place the pressure sensors, such as the pitot tube, in a portion of the pipe which extends a number of (typically 10) pipe-diameters upstream from the point of measurement and a number of (typically 5) pipe-diameters downstream from that point. The resulting multiple-pipe-diameter section is referred to as a "meter-run" and must be provided and placed in series with the pipe in which gas flow is to be measured.
An orifice meter comprises an orifice plate that is interposed in a pipe by dividing the pipe into sections with ends which abut opposite faces of the orifice plate. The latter has an orifice through its center, and the pipe sections have openings, or "taps," through their sides. Gas flow through the compound pipe comprising the sections is measured by monitoring the pressure differential between the taps respectively upstream and downstream of the orifice plate. The same rule of multiple pipe-diameters upstream and downstream from the point of measurement applies to the orifice meter as well as to the velocity pressure metering.
All three common techniques for measuring gas flow involve a substantial length of pipe that must be free from bends or any other types of obstruction. Room for such a length of straight, unobstructed pipe may be difficult to come by in a landfill. This is particularly so because, in order to protect the instruments connected to the "meter-run," it would be desirable to enclose it in a vault box, which is normally installed over the wellhead, to limit access to the flow control valve, which is usually placed at the wellhead.